Enzymes in milk

Irradiation. Although no irradiation systems for pasteurization have been approved by the U.S. Food and Dmg Administration, milk can be pasteurized or sterilized by P tays produced by an electron accelerator or y-rays produced by cobalt-60. Bacteria and enzymes in milk are more resistant to irradiation than higher life forms. For pasteurization, 5000—7500 Gy (500,000—750,000 tad) are requited, and for inactivating enzymes at least 20,000 Gy (2,000,000 rad). Much lower radiation, about 70 Gy (7000 tad), causes an off-flavor. A combination of heat treatment and irradiation may prove to be the most acceptable approach. 

As many as 60 indigenous enzymes have been reported in normal bovine milk. With the exception of a-lactalbumin, which is an enzyme modifier in lactose synthesis (Chapter 2) most, if not all, of the indigenous enzymes in milk have no obvious physiological role. They arise from three principal sources ... 

Deterioration (lipase (commercially, probably the most significant enzyme in milk), proteinase, acid phosphatase and xanthine oxidase) or preservation (sulphydryl oxidase, superoxide dismutase) of milk quality. 

The distribution of the principal indigenous enzymes in milk and their catalytic activity are listed in Table 8.1. In this chapter, the occurrence, distribution, isolation and characterization of the principal indigenous enzymes will be discussed, with an emphasis on their commercial significance in milk. 

Other proteinases. The presence of low levels of other proteolytic enzymes in milk has been reported (see Fox and McSweeney, 1996). Most of these originate from somatic cells, and their level increases during mastitic infection. The presence of cathepsin D, a lysozomal enzyme, in milk suggests that all the lysozomal proteinases are present in milk although they may not be active. These minor proteinases are considered to be much less significant than plasmin, but more work on the subject is necessary. 

The occurrence of a peroxidase, lactoperoxidase (LPO), in milk was recognized as early as 1881. It is one of the most heat-stable enzymes in milk its destruction was used as an index of flash pasteurization (now very rarely used) and is now used as an index of super-HTST pasteurization. 

Table 8.4 Partial list of minor enzymes in milk (modified from Farkye, 1992)... 

Superoxide dismutase (SOD), an indigenous enzyme in milk, was discussed in section 8.2.10. A low level of exogenous SOD, coupled with catalase, was shown to be a very effective inhibitor of lipid oxidation in dairy products. It has been suggested that SOD may be particularly useful in preserving the flavour of long-life UHT milk which is prone to lipid oxidation. Obviously, the commercial feasibility of using SOD as an antioxidant depends on cost, particularly vis-d-vis chemical methods, if permitted. 

Bjorck, L. (1993) Indigenous enzymes in milk Lactoperoxidase, in Advanced Dairy Chemistry, Vol. 1 Proteins, (ed. P.F. Fox), Elsevier Applied Science, London, pp. 332-9. 

Shovers, J., Fossum, G. and Need, A. 1972. Procedure for electrophoretic separation and visualization of milk-clotting enzymes in milk coagulants. J. Dairy Sci. 55, 1532-1534. 

Two types of enzymes in milk are important those useful as an index ol heat treatment and those responsible tor bad flavors. Phosphatase is destroyed by the heat treatments used to pasteurize milk hence its inactivation is an indication of adequate pasteurization. Lipase catalyzes the hydrolysis of milk fat which produces rancid flavors. It must be inactivated by pasteurization or more severe heat treatment to safeguard the product against off-flavor development Other enzymes reported to have been found in milk include catalase, peroxidase, protease, diastase, amylase, oleinase. reductase, aldehydrase. and lactase. 

The virtual lack of adenine aminohydrolase in animal tissues has been confirmed in several laboratories (21-25). The reported presence of this enzyme in milk (21) has not been confirmed (26). Evidence for adenine aminohydrolase in Saccharomyces cerevisiae and Candida utilis based on enhanced growth on adenine (27) has been supported by Rousch and his colleagues (28, 29). The direct deamination of adenine by extracts of E. coli (30-32) has not been verified (33). 

Griffiths (2) studied the thermal stability of the naturally occurring enzymes in milk at temperatures between 65 and 80°C, in order to choose an enzymatic index of adequate pasteurization of milk. For the acid phosphatase, the D-values of 7.38 min at 75°C and 7.87 min at 80°C corresponded to a z-value of 6.6°C for the inactivation of lactoperoxidase, D-values of 0.80 min at 75°C and 0.075 min at 80°C corresponded to a z-value of 5.4°C for amylase (saccharifying activity), D-values of 0.85 min at 75°C and 0.45 min at 80°C corresponded to a z-value of 16.2°C. The naturally occurring enzymes in milk showed lower D-values than those found for GFPuv in the three buffers. However, a z-value of 16.64°C characterized for GFPuv in phosphate buffer solutions at pH 7.0 was similar to that obtained for amylase in milk. 

In this chapter, the lipolytic enzymes in milk and milk products and the causes, consequences and assessment of their action are discussed. The significance of lipases in human nutrition and in the production of characteristic flavors in certain dairy products is also covered. 

Olivecrona, T., Bengtsson-Olivecrona, G. 1991 Indigenous enzymes in milk.-II Lipase. In Food Enzymology (P.F. Fox, ed.), pp. 62-78, Elsevier Applied Science, London. 

Sorhaug, T., Stepaniak, L. 1997. Psychrotrophs and their enzymes in milk and dairy products quality aspects. Trends Food Sci. Technol. 8, 35-41. 

The less severe heat treatment of modern milk powder production can lead to problems since enzymes present in the milk are not inactivated. The enzyme in milk products, particularly milk powder, that causes problems is lipase. It should be appreciated that this is not the native... 

The highest concentration of xanthine oxidase in mammals is found in the milk and lactating mammary gland [78, 92], although it is not known whether all species secrete the enzyme in milk. For obvious reasons, nearly all the information available refers to xanthine oxidase from bovine milk, but in fact both rat and guinea-pig milk have a higher enzyme content [92]. Table 3.2... 

Rademacher, B. and Kessler, H.G. 1997. High pressure inactivation of microorganisms and enzymes in milk and milk products. In K. Heremans (Ed.), High Pressure Research in Biosciences and Biotechnology, pp. 291-293. Leuven, Belgium Leuven University Press. 

Status of opioids in bovine milk and its influence on enzymes in milk synthesis Hossain, M. Afzal Kumar, Amalendra Ganguli, N. C. 

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